In conventional manner, such boilers are fitted with burners associated with air feed means, said burners being optionally tiltable. Boilers are adjusted firstly by adjusting the flow rate of recycled gas, so as to increase the flow of flue gas in the boiler and increase heat exchange with equipment that operates in convection, and secondly by changing the level of the fire by means of the tiltable burners which act on equipment that works by radiation. Using these two means, it is possible to adjust the superheating and re-superheating absorption by an appropriate disposition of equipment in the hearth.
In known boilers, the recycled gases are conveyed by ducts which pass through the lateral screens of the boiler. This prior art is illustrated by American patents U.S. Pat. Nos. 3,090,332 and 2,985,152.
More precisely, with boilers burning fuel oil, these ducts are connected in substantially horizontal manner to one of the slopes of the ash box (where the slopes are screens of heat exchanger tubes that slope downwards and inwards at the bottom of the boiler to form the ash box).
For a boiler that burns coal or a mixture of coal and fuel oil, these horizontal ducts open out into the screen that forms one of the side faces of a clinker-removing ash hopper disposed beneath the ash box.
It is important to obtain good uniformity in the gases recycled through the boiler in order to avoid disturbing the combustion vortex established level with the burners.
That is why it is essential to install a plurality of ducts feeding recycled gases to any one side face, there being up to four ducts for a large hearth. Unfortunately making the openings through the screens of heat exchanger tubes requires operations that are relatively complex, in particular for so-called "spiral" tube screens, and arrangements for ensuring tube continuity. Furthermore, when of large dimensions, these openings have bottom points that cannot be emptied.
In addition, with such a lateral disposition of recycled gas feed ducts, it is never possible to install the ducts in a rational configuration because of the equipment (in particular the manifolds and the feed tubes extending the screens of heat exchanger tubes) that are always present and that take up a large amount of room on the sides of the boiler, and that necessarily require the recycling ducts and the heat exchanger tubes to be diverted and special gaskets to be installed, which operations are particularly expensive.
Finally, when burning coal, there is always the problem of ash accumulating at the outlets of the recycled gas feed ducts, even in boilers having an ash hopper, since the ducts are horizontal where they open out into the hopper.